Isolation transformers for computer data interface
From my reading of various data interface projects, the higher the quality of transformer used to isolate the radio from the computer the better will be the operation of the interface. I can't/won't afford Jensen or other high-end transformers; the lowest cost of those brands was in the neighborhood of $35 apiece. I happened upon a company that sold/manufactured what appeared to be high quality transformers from the specifications presented that were around $10 dollars or so. I finally decided to build an interface but I have lost my hyperlink to that company. Does anybody have any suggestions other than DigiKey or Mouser or possibly know of the company I stumbled into? I have tried numerous Internet searches to no avail.
I know I could buy a Rig Blaster or similar; however, I want to be able to tinker with the interface.
From my reading of various data interface projects, the higher the quality of transformer used to isolate the radio from the computer the better will be the operation of the interface. I can't/won't afford Jensen or other high-end transformers; the lowest cost of those brands was in the neighborhood of $35 apiece. I happened upon a company that sold/manufactured what appeared to be high quality transformers from the specifications presented that were around $10 dollars or so. I finally decided to build an interface but I have lost my hyperlink to that company. Does anybody have any suggestions other than DigiKey or Mouser or possibly know of the company I stumbled into? I have tried numerous Internet searches to no avail.
I know I could buy a Rig Blaster or similar; however, I want to be able to tinker with the interface.
I'm not sure the premise is sound. There are plenty of transformers in the $2-$6 range that should work very well. Most transformers designed for a modem would probably work fine in this application.
__________________ "FCC whacker": "A wannabee FCC agent who lives to find violations (real or imagined) anywhere he can. He will use the most narrow definition of the rules, if it suits him. He wants to enforce his interpretation of the FCC rules onto everyone."
Check out one of these at your local Wal-Mart: http://www.walmart.com/catalog/produ...duct_id=896355. You can use it for isolating your audio right out of the package -or- you can remove the two transformers and build your own interface.
A lot of the $2-$6 transformers perform fairly well and people have had good luck with them there is more chance of them being problematic. As more complex modulation schemes are used, the distortion introduce by the type of materials and the bandwidth of the lower end transformers just adds to the signal distortion that will be created in the transceivers audio path. This is not some of that flim-flamery that you see in the high-end audio world where cables and connectors are accused of causing problems but real, measurable problems. Getting a little better transformer tends to take that factor out of the "equation" of errors. However, now that you mention it, Jameco may be where I took a lead for the transformers I was thinking of.
A lot of the $2-$6 transformers perform fairly well and people have had good luck with them there is more chance of them being problematic. As more complex modulation schemes are used, the distortion introduce by the type of materials and the bandwidth of the lower end transformers just adds to the signal distortion that will be created in the transceivers audio path. This is not some of that flim-flamery that you see in the high-end audio world where cables and connectors are accused of causing problems but real, measurable problems. Getting a little better transformer tends to take that factor out of the "equation" of errors. However, now that you mention it, Jameco may be where I took a lead for the transformers I was thinking of.
Thanks for the suggestion!
Bill, WD5HHH
This might be a good place to try an experiment. Buy a couple of the expensive ones, and a couple of the cheaper versions. Build your interface so that you can substitute the transformers out. Then try several days or months using each of the setups.
I would bet that you will see no difference. These transformers work in multi-KB phone modems which have much more complex modulation schemes than we have on HR.
In reality, for most interface applications the transformers are not needed. Just a cable from the computer to the radio. Many hams use that sort of set up just fine.
Joe
__________________ "FCC whacker": "A wannabee FCC agent who lives to find violations (real or imagined) anywhere he can. He will use the most narrow definition of the rules, if it suits him. He wants to enforce his interpretation of the FCC rules onto everyone."
Jensen transformers are for applications to send mic level audio signals over 30 feet, 100 feet cable runs with minimum hum and extremely wide dynamic range (110 dB for live music events) spanning from sub hertz to 50 MHz. Long cable length creates some interesting problems, like the noise generated if some body steps on to the cable or the cable gets pulled, as the capacitance between center conductors and the outer shield changes generating common mode noise.
Those expensive transformers use special alloy to reduce distortion and computer designed winding layout to ensure maximum common-mode noise rejection ratio (CMRR - better than 100 dB for best transformers).
Also there are special mic/guitar/audio cables that using Kevlar fiber (CANARE www.canare.com) or conducting plastic wrapping (MOGAMI www.mogamicable.com) to fight stress induced noise.
Typical ham radio setup only needs 300 Hz to 3,000 Hz over a few feet of distance. Any modem transformer will do just fine.
"Most transformers designed for a modem would probably work fine in this application"
That could be a BAD IDEA, depending on the set up! Most modem transformers are 600/600 ohm isolation designs. Although 600 ohms may work fine on the computer side, they are bad for the radio! Most radio's with data or accessory ports have output impedances of 6,000 to 10,000 ohms. Hanging a 600 ohm transformer across one of these will severly load down the audio stages.
So if you're planning on using only the radio's speaker (or headphone) jack and the mic port, and the computer's speaker and mic port, you can probably get away with surplus modem transformers. However, if you are planning on using the radio's DATA or ACC port, and the sound card's LINE OUT port- which it the preferered connection- they are unacceptable and will severly load down the radio and sound card.
A much better solution would be to use a pair of some type of audio output transformers, with a 5K - 10K impedance on one side, and an 8 ohm or higher impedance on the other side. The high impedance windings always connects to the signal sources (outputs), and the low impedance windings connect to the signal destinations (inputs), as shown below:
RADIO OUTPUT> HI Z WINDING || LOW Z WINDING > SOUND CARD INPUT
COMPUTER OUTPUT > HI Z WINDING || LOW Z WINDING > RADIO INPUT
I built a digital interface for my IC718 and used transformers but after building it I asked myself what are the transformers actually doing. I determined the answer was nothing! The IC718 uses a single ground on their accessory connector which is all you need for the interface. Unless your radio uses separate grounds for functions like PTT, power, or audio a transformer is unnecessary.
Here is link and in it somewhere is my final PSK31 interface schematic.
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"Most transformers designed for a modem would probably work fine in this application"
That could be a BAD IDEA, depending on the set up! Most modem transformers are 600/600 ohm isolation designs. Although 600 ohms may work fine on the computer side, they are bad for the radio! Most radio's with data or accessory ports have output impedances of 6,000 to 10,000 ohms. Hanging a 600 ohm transformer across one of these will severly load down the audio stages.
Don't know where you are getting this info. I haven't seen any modern radio that has an aux output impedance that high. Most radios are in the 300-600 ohm range.
Quote:
So if you're planning on using only the radio's speaker (or headphone) jack and the mic port, and the computer's speaker and mic port, you can probably get away with surplus modem transformers. However, if you are planning on using the radio's DATA or ACC port, and the sound card's LINE OUT port- which it the preferered connection- they are unacceptable and will severly load down the radio and sound card.
I haven't seen that at all. In fact, Buxcom who makes and sells lots of these units each year, uses 600:600 transformers on both sides of their audio interface.
Quote:
A much better solution would be to use a pair of some type of audio output transformers, with a 5K - 10K impedance on one side, and an 8 ohm or higher impedance on the other side. The high impedance windings always connects to the signal sources (outputs), and the low impedance windings connect to the signal destinations (inputs), as shown below:
RADIO OUTPUT> HI Z WINDING || LOW Z WINDING > SOUND CARD INPUT
COMPUTER OUTPUT > HI Z WINDING || LOW Z WINDING > RADIO INPUT
If you try that, you probably won't end up with enough audio to drive the sound card. It might work, though.
Why not just use what others have used in the past and have proved that they work? No need to make this overly complex. It isn't rocket science. The input to the computer is near 1000 ohms. The output from the radio is near 300 ohms. A 600 ohm transformer is a good match. Same on the other side. The computer output is near 30 ohms. The HR mike input is around 600 ohms. The 600 ohms transformer will work well for that, too.
Joe
__________________ "FCC whacker": "A wannabee FCC agent who lives to find violations (real or imagined) anywhere he can. He will use the most narrow definition of the rules, if it suits him. He wants to enforce his interpretation of the FCC rules onto everyone."
I built a digital interface for my IC718 and used transformers but after building it I asked myself what are the transformers actually doing. I determined the answer was nothing! The IC718 uses a single ground on their accessory connector which is all you need for the interface. Unless your radio uses separate grounds for functions like PTT, power, or audio a transformer is unnecessary.
Here is link and in it somewhere is my final PSK31 interface schematic.
Yes, that's the simple way that many hams use. The transformers are really not needed at all. The only thing really needed is a way to match audio levels.
Joe
__________________ "FCC whacker": "A wannabee FCC agent who lives to find violations (real or imagined) anywhere he can. He will use the most narrow definition of the rules, if it suits him. He wants to enforce his interpretation of the FCC rules onto everyone."
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Isolation transformers for computer data interface
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